H. Anthony Chan and Paul J. Englert

Transcription

1 Chapter INTRODUCTION H. Anthny Chan and Paul J. Englert 1.1 SYNOPSIS OF RELIABILITY TRENDS AND AIM OF BOOK This handbk desribes a set f Best Praties fr utilizing aelerated stress testing (AST) as part f the methdlgy fr systematially imprving the hardware reliability f manufatured prduts. A key feature f this methdlgy is t apply stress testing tehniques t a prdut early in its life and then rretive atin t fix the prblems fund as quikly as pssible. A main gal f aelerated stress testing is t identify design defiienies r prblems with mpnent quality r manufaturing presses in rder t ahieve a mre reliable prdut as prdutin vlumes inrease. The emphasis f this reliability imprvement methdlgy is n identifying and eliminating reliability prblems, nt n sreening ut defets. Hwever, sine it may take sme time t eliminate ertain prblems, the AST Best Praties als seek t ptimize the appliatin f nging stress testing (that is, burn-in and/r temperature yling) where apprpriate s as t minimize test sts and test intervals. This bk is riented prinipally tward the authrs' bakgrunds, the eletrnis industry. The methds presented here may als be applied t ther types f prduts. Befre preeding with ur disussin f the aelerated stress testing methdlgy, it is useful first t nsider the status f reliability within the eletrnis industry, thereby establishing the business ntext within whih reliability enhanement prgrams must exist. The remainder f the Intrdutin utlines the ntents f the handbk. The eletrnis industry generally regnizes the desirability f "high" prdut reliability, but there are als strng business pressures that tend t mprmise prdut reliability. Cnsequently, it is useful first t nsider reliability frm a realisti business perspetive. Failure t prvide adequate reliability an be stly t a business in several ways. Mst immediately, there is the st f rewrking r srapping gds within the manufaturing envirnment. In additin, pr prdut quality r lak f rbustness an lead t exessive mpnent and prdut inspetin r test sts. After prduts are shipped, the st f pr quality r reliability an munt rapidly. Warranty repairs may nt nly be expensive t the manufaturer but are ften far mre stly t the ustmer in terms f lss f servie r inreased maintenane sts. Finally, a lak f prdut reliability an have a signifiant impat n future sales. On the ther hand, a business may inur exessive expenses pursuing very high reliability standards with very little paybak (thugh in ur experiene this rarely urs). The st f imprving r maintaining prdut reliability must be weighed

2 Chapter 1 Intrdutin against the benefits derived, and it shuld be regnized that frm a business perspetive there is an adequate, thugh less than perfet, level f reliability apprpriate fr any prdut. Of urse, the apprpriate level f quality des depend n the nature f the prdut and hw the ustmer uses it. On ne end f the spetrum, very high reliability is required fr high-end mputing r mmuniatins systems that are imprtant t a ustmer's revenue stream. On the ther end, relatively simple but innvative prduts that have a high degree f ustmer appeal may be able t prsper fr a time even if they are nt very reliable. A business must understand the st f reliability, bth diretly t its wn peratins and indiretly t its ustmers and their willingness t make future purhases, and must have a plan fr ahieving an apprpriate level f reliability fr its prduts. Unfrtunately, hwever, this is ften nt the ase. Several nging trends in the eletrnis industry pse a hallenge t thse seeking t prvide adequate prdut reliability nsistently. The fast pae f inreased silin integratin and inreased devie speed, upled with an assiated trend tward miniaturizatin, are leading t prduts with ever higher funtinal apability and mplexity pakaged in smaller vlumes and ften at inreased pwer densities. This has been ampanied by a fairly rapid evlutin in pakaging tehniques, materials, and manufaturing presses. The rapid imprvement in the funtinality f eletrni system mpnents, tgether with the sharp drp in the assiated st f funtinality, has reated nsumer demand fr prduts ffering ever mre features and apabilities. This in turn has led t a rapid pae f new prdut develpment and existing prdut bslesene. This is partiularly true fr the nging nvergene f mmuniatins, mputing, and nsumer prduts, whih represents yet anther majr trend in the eletrnis industry. This nvergene, alng with a strng mve tward glbal markets, is leading t further widening f the envirnmental nditins under whih a prdut is expeted t perate. This piture f rapid-paed develpment f new prduts f ever inreased mplexity perating in diverse glbal envirnments, upled with intense mpetitive pressures, has led t a strng emphasis in the eletrnis industry n bringing a ntinual stream f new prduts t market in a very shrt time while at the same time maintaining a lw st f gds t assure adequate prfit margins. In respnse t the time and st pressures f the market, businesses frequently take a rather minimal apprah t reliability, attempting t establish a minimum level f prdut testing r inspetin that will prvide a level f reliability they feel is just adequate fr the market they serve. Hwever, given the manner in whih mst mpanies rganize their prdut and funtinal respnsibilities, the ttal st f reliability t the business is ften underestimated r unregnized, and lal ptimizatin in reduing sts tends t lead t less than desirable reliability results. While this bk des nt delve int the methds by whih a business shuld ptimize its verall apprah t quality and reliability, the reliability imprvement methdlgy advated in this bk des require a prative management attitude tward reliability. Management must be willing t invest in the early detetin and eliminatin f reliability prblems with the expetatin that they will be rewarded with lwer "dwnstream" reliability sts and enhaned sales thrugh imprved ustmer satisfatin. In this bk, we assume that management has already established the traditinal quality assurane prgrams fr mpnents and manufaturing presses and has instituted rbust design methds t sme degree. Hwever, given the ever inreasing

3 Setin 1.2 Bakgrund f Military and Industrial Stress Testing Praties 5 mplexity f new prduts, the faster shedules n whih they are develped, and the need t be at r near the leading edge in new tehnlgies r manufaturing presses, there is still a need fr additinal prative presses t assure prdut reliability. The main purpse f this bk then is t prvide the means t quikly and enmially assess the ptential reliability f a prdut and affrd the pprtunity t fix reliability prblems early befre they lead t majr lsses. Ding aelerated stress testing is nt heap, and the st f the assiated failure mde analysis and rretive atin may be even mre expensive. Typially, these sts an nly be justified by regnizing the fullstream st f prdut reliability ver the life f a prdut. Hwever, in a number f well-rganized business peratins where the full-stream st f reliability has been understd, the aelerated stress testing methdlgy disussed in this bk has been fund t be f great value. 1.2 BACKGROUND OF MILITARY AND INDUSTRIAL STRESS TESTING PRACTICES In 1992, a tehnial mmittee n Envirnmental and Reliability Testing was frmed under the Cmpnent, Pakaging, and Manufaturing Tehnlgy Siety (CPMT) f the Institute f Eletrial and Eletrnis Engineers (IEEE). This mmittee prmtes the free tehnial exhange f stress testing praties and experienes withut releasing prprietary prdut infrmatin. This mmittee als initiated the annual IEEE Wrkshp n Aelerated Stress Testing in 1995 [1]. In 1997, the IEEE Reliability Siety (RS) jined effrts with the IEEE CPMT Siety t -spnsr this wrkshp [2]. In additin, the Sreening and Testing Cmmittee f the IEEE RS has als expanded its ativities, shifting its fus frm nventinal sreening t aelerated stress testing. The need t lwer sts and t take advantage f varius reliability effrts in the eletrnis industry has reently prmpted the U.S. military t begin a series f effrts t phase ut military standards and t purhase Cmpnents Off The Shelf (TS). The eliminatin f military standards des nt diminish the need t strive fr reliability; rather, it leaves pen the means t ahieve reliability. Withut utilizing military speifiatins and standards, alternatives are needed t ensure that prduts are reliable, while their sts are kept dwn. That part f the eletrnis industry that has been relying n military reliability standards is als affeted in the same way. The spe f Envirnmental Stress Sreening fr the military and spae appliatins has als mved twards AST as the U.S. gvernment buys mpnents frm mmerial vendrs. The envirnmental stress sreening fr eletrni hardware (ESSEH) guidelines issued by the Institute f Envirnmental Sienes and Tehnlgy (IEST) has als attempted t expand its spe t enmpass mre prgressive frms f AST [3]. Fr the telemmuniatins industry, a set f guidelines n Envirnmental Stressing was issued by Telrdia Tehnlgies (the frmer Bell Cmmuniatins Researh Cmpany - Bellre), whih had been the researh and develpment arm fr the reginal telephne mpanies in Nrth Ameria [4]. Starting in 1994, the Natinal Center fr Manufaturing Siene (NCMS) has als been spnsring a team f eletrni equipment manufaturers t evaluate the effetiveness f ertain AST methds [5].

4 6 Chapter 1 Intrdutin 1.3 OVERVIEW OF THE AST HANDBOOK This handbk is divided int a number f hapters dealing with varius aspets f aelerated stress testing (AST). Chapter 2 prvides bakgrund n the nept f aelerated stress testing and lays the fundatin fr later disussins f the mre pratial aspets f the implementatin f AST prgrams. The hapter begins with a ratinale fr aelerated stress testing based n the bservatin that many reliability prblems are aused by inadequate design margins r variatins in manufaturing presses r mpnent quality. T reate a reliable prdut, ne must ahieve rbust design margins and tighten ntrl f prdut variatins. We bserve that AST prgrams are f value hiefly in identifying and eliminating prdut defiienies early in a prdut's life. Stress testing must be upled with rt ause analysis (RCA) and rretive atin t ahieve prdut rbustness. We als explain that there are limitatins t what an be ahieved with stress testing, depending n the types f failure mdes present in a prdut. We then disuss a series f tehnial issues regarding the implementatin f AST. These tehnial issues inlude identifying the prinipal mdes f perfrming AST and hw they relate t eah ther, as well as milder frms f stress testing. We review the variety f stress stimuli that an be used in stress testing, and we nte the types f failure mehanisms that they are mst effetive at addressing. We then explain hw t determine an apprpriate level f stress t use in an AST prgram and hw t asertain that the applied stresses are safe and d nt damage gd prduts. We als disuss apprahes fr better deteting faults stimulated by stress testing and prvide preferenes n the frm f prdut t be tested. The enmi benefits f AST and qualitative guidelines fr ptimizing the st effetiveness f an AST prgram are disussed at the end f the hapter. Chapter 3 presents a set f Best Praties fr perfrming aelerated stress testing. We remmend a strategy fr implementing aelerated stress testing s as t ahieve maximum benefit. We explain that a number f ptins are available and that the develpment f an ptimal AST prgram fr a given prdut will depend n the nature f the prdut itself (hw rbust r unreliable it is, and what the prinipal failure mdes are) and n the reliability bjetives set fr the prdut. AST is really an iterative press in whih reliability data gathered at ne stage f stress testing shuld be used t determine what shuld be dne in a subsequent stage. We define the prinipal implementatin mdes fr AST as fllws: Design AST Manufaturing Qualifiatin AST Peridi Qualifiatin AST Prdutin Sampling AST Full-Prdutin AST Eah f these implementatin mdes is desribed as a mar-press nsisting f a series f generi subpresses. The generi subpresses, whih may be mmn t mre than ne f the abve mar-presses, are: Plan Prgram Baseline Prdut

5 Setin 1.3 Overview f the AST Handbk 7 Take Crretive Atin Develp Manufaturing AST Regimen Demnstrate Safety f AST Regimen Perfrm Manufaturing AST Optimize Manufaturing AST Regimen In presenting this methdlgy, a prinipal remmendatin and wrking assumptin is that reliability imprvement effrts an best be arried ut within the framewrk f a frmally defined reliability prgram nduted by a multidisiplinary team. Suh a prgram shuld inlude ativities ranging frm planning the prgram and nduting the tests n thrugh rt ause analysis f failures and implementatin f rretive atins t eliminate prblems fund. These diverse ativities require the partiipatin f a variety f team members with mpnent, design, test, and manufaturing respnsibilities. In Chapter 4 we desribe these generi subpresses as a detailed series f tasks t be perfrmed by a multidisiplinary team nduting a frmal reliability prgram. Mst f these tasks need t be arried ut t perfrm an effetive prgram, even if the prgram is nt as rigrusly defined as in this hapter. Chapter 5 prvides a set f Guidelines fr Design and Manufaturing AST Qualifiatin. Here we disuss in greater detail sme f the mre pratial issues that are mmnly enuntered in planning and nduting an AST prgram, and we desribe speifi apprahes and tehniques that have been fund t be partiularly useful in testing eletrni prduts r systems. Chapter 6 desribes sme methds that prvide a basis fr ptimizing aelerated stress testing prgrams. This hapter als presents sme simple neptual mdels that prvide an imprved understanding f varius basi issues that are ften raised in the eletrnis industry with regard t the effetiveness f aelerated stress testing. Chapter 7 prvides a theretial basis fr estimating the apprpriate number f prdut samples that shuld be subjet t AST in rder t ahieve a reliability gal. Chapter 8 presents an verview fr failure analysis fr eletrial mpnents. Chapters 9 thrugh 13 present a pratial verview f equipment and tehniques that have been mmnly utilized fr aelerated stress testing. The types f stresses addressed in these hapters inlude air-based thermal, liquid-based thermal, vibratin, and shk. Chapter 14 desribes the Guidelines fr AST Safety Qualifiatin. This hapter desribes the srt f wrk that is perfrmed t demnstrate that the stress levels used fr a seleted AST regimen are safe and d nt damage gd prduts. We disuss tw prinipal apprahes t safety qualifiatin, namely, the diret safety qualifiatin f a fully assembled prdut and the mre generi qualifiatin f the individual mpnents that g int the prdut. Chapters 15 and 16 desribe helpful ways fr AST pratitiners t plan AST experiments and t analyze resultant failure data. Chapters 17 thrugh 21 prvide sme exellent ase studies that illustrate the appliatin f aelerated stress testing in the mputer, mmuniatins, and nsumer prdut industries. Beause f the prprietary nature f muh f the failure data btained in reliability studies, muh f the dumented wrk n aelerated stress testing appears nly in internal mpany publiatins and is nt available fr general presentatin t the publi.

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7 Referenes Chapter 22 illustrates hw benhmarking tehniques may be applied t AST. Table 1.1 shws the type f AST demnstrated in eah f eah ases. This table als gives the different arnyms used by the individual authrs when they refer t the different AST presses. The ases in this bk were seleted fr a variety f reasns. Sme were hsen beause they are frequently ited in the literature due t the seminal nature f the wrk, thers beause f the dramati imprvements realized by diligent usage f AST, and still thers beause f their unique r nvel apprah t stress testing. A glssary f stress testing terminlgy is prvided fllwing Chapter 22. Sme f the terms used in the field f stress testing may be nfusing even t experiened reliability engineers beause f the many variatins in usage. We have attempted t delineate sme f the mst ften used terminlgy in the glssary. A bibligraphy f ver 250 publiatins is presented at the end f the handbk. Prir t the 1990s, publiatins n stress testing were sparse, primarily beause f the prprietary nature f muh reliability wrk. And mst f the prir published material was devted t stress sreening. In the early 1990s, publiatins in stress testing began t blssm with the wave f interest in imprved quality and reliability. Tday, papers n stress testing methds and results are appearing regularly in IEEE and IES publiatins and nferene preedings. REFERENCES [1] Preedings f the 1996 Wrkshp n Aelerated Stress Testing, Otber 17-18, 1996, Ottawa, Ontari, Canada. [2] Preedings f the 1997 Wrkshp n Aelerated Stress Testing, Otber 15-17, 1997, Dallas, T. [3] Envirnmental Stress Sreening Guidelines fr Assemblies (1990), Prepared by the IEST ESSEH Tehnial Cmmittee, [4] Generi Requirements fr Envirnmental Stressing Applied t Telemmuniatins Prdut, Dument N. GR-2840, Telrdia Tehnlgies, [5] Envirnmental Stress Sreening Final Reprt, Reprt N. 0006RE 97, Natinal Center fr Manufaturing Sienes, April, 1997.